function fakeAsync(id){
  const start = Date.now(),
        delay = Math.random()*1000;
  return new Promise((resolve, reject) =>{
    setTimeout(function(){
      const end = Date.now();
      console.log(`fakeAsync use ${end - start}ms`);
      report(id, end-start);
      resolve(end - start);
    },delay);
  });
}

function report(id, cost){
  console.log(`#${id} task done. use ${cost} ms .\n`);
}
module.exports = fakeAsync;
